‘A cocktail party in concrete’, according to Tatler, but the M4 is not always a pleasure to drive. Whose fault is that? Photograph: Rex

The crowds were already waiting in Margam, on the outskirts of Port Talbot. It was the eve of Ukip’s first Welsh conference, and along with the journalists and the TV cameras and the protestors waving inflatable rats, 100 or so party members had congregated at a special reception for their leader, Nigel Farage. Alas, when Farage finally appeared, having scuttled all the way from London, he was two hours late; the reception was all but over.

But Farage remained undimmed: “It took me six hours and 15 minutes to get here,” he told the BBC’s Sunday Politics Wales a couple of days later. “It should have taken three-and-a-half to four. That has nothing to do with professionalism. What it does have to do with is a country in which the population is going through the roof, chiefly because of open-door immigration, and the fact the M4 is not as navigable as it used to be.”

This seemed like a thesis worth testing, so Sarah, the photographer, and I set off on the M4 at midday on Wednesday, eyes peeled for immigrants. The Romans once followed this route, so there is a precedent, of course. But without the Romans we wouldn’t have sewers or windows or even an M4 at all, so it’s probably for the best that we waved them through border control back in 43AD.

As Farage attested, this is a journey that should take well under six-and-a-quarter hours. A quick consultation of Google maps promises that, without traffic, it should take just over two- and-a-half hours – although today it cautions the trip could be 15 minutes longer, which I assume is due to a mass swarm of Armenians somewhere near Bath.

The M4 is Britain’s second-oldest motorway. It runs for 180 miles, from Chiswick in west London to a point 12 miles south-east of Carmarthen in west Wales, passing through Reading, Swindon, Bristol, Cardiff, Port Talbot and Swansea along the way.

If you’re coming from London, it’s the A4 at first, carrying you west along the Golden Mile stretch of art deco buildings and historical landmarks – the famous Lucozade building, with its animated illuminated sign, the Coty Cosmetics factory, the Gillette factory and the old Currys office.

At the Chiswick flyover, it becomes the M4 proper, an elevated dual carriageway that cost £6m to build and was designed to alleviate the crush of 40,000 cars using the junction at Chiswick roundabout. It was opened in September 1959, at a ceremony overseen by the actress Jayne Mansfield, and five years later it became part of the motorway. A decade after it was opened, it could lay claim to being the most dangerous road in Britain, and by 2009, 97,000 vehicles were said to be using it every day.

Certainly, it is busy in west London this lunchtime: the roads bristle with that kind of dusty, snarled-up city traffic; cars find themselves suddenly detoured and disorientated, wary of bus lanes and traffic cameras; cyclists weave past juggernauts; motorbikes curl their lips at traffic lights. But there’s a kind of glee once we reach the flyover, the sense of being set loose from the city.

We aren’t travelling particularly fast – Heathrow-bound traffic ducks and dives between lanes, but there is a pleasing sense of momentum.

‘The M4 is no posher than our other motorways. And none of them are as good as Scandinavian roads’ … sales manager Ed Smith at Reading services. Photograph: Sarah Lee/Guardian

We stop at Heston services, the M4’s first service station. There is a WH Smith, a Costa Coffee, a Greggs, a Travelodge, a small collection of slot machines and arcade games and vending machines offering bags of Aero Bubbles, Red Bull and Coke. A Christmas tree stands in the atrium. “Welcome to Winter Whopperland” beams the sign outside Burger King.

I speak to Pami, behind the counter in WH Smith, to ask her just how many non-Brits come through Heston. She smiles nervously over the giant bars of Dairy Milk. “Loads,” she says. “Lorry drivers, bus drivers, all kinds of people. ’Cause they’re travelling, isn’t it?”

It’s the Costa that’s busiest today, full of people in suits – mostly male, mostly non-white – holding informal meetings or reading through documents. Gagan Deep Noor, 29, has been the manager here for four years. It is busiest, he says, at mornings, lunchtimes and weekends, a mingling of “businesspeople and regular people coming from London, or tourists”. But mostly, he says, his customers are British. He looks over at a group of eight people, bent over their coffee cups, deep in a meeting. All of them are Asian. “It’s a multicultural country,” he says. “Which is good, because we learn so many things from people.”

On we drive. The road is freeing up now, the landscape softening, the winter sun pressing sharply through the bare branches of the roadside trees. We are counting the Downton lorries and Waitrose vans, following signs for “The West”, nodding at the signs for Windsor and Eton and Slough.

The crucial thing about the M4 is that it is, as motorways go, one of Britain’s loveliest. It takes you through some of our most sumptuous countryside, past some of our major public schools, Eton and Marlborough, and the country piles of Gloucestershire. Tatler once named it the country’s “poshest motorway … a cocktail party in concrete”.

Tom Fort, author of The A303: Highway to the Sun, remembers the M4 being built, but holds it in no great affection, although there is one idiosyncrasy of which he’s fond. “There’s this weird thing about the signs warning you to watch out for deer,” he says. “There’s one – I think it’s heading west – that warns of the danger of encountering deer for the next 37-and-a-half miles. But going the other way it says 33 miles, which suggests there are four and a half miles of the M4 were there are only deer in one direction.”

We wait in the bright cold outside Reading services. Inside, there’s a Marks & Spencer and an Upper Crust, and an AA representative jigging from one foot to the next calling “Hello, missy! Hey! Hey!” as we pass. Outside, there’s another Greggs and a West Cornwall Pasty Company stand. Ed Smith walks out through the automatic doors in a suit and red tie, carrying a plastic bag. He is a 53-year-old sales manager on his way from Potters Bar in Hertfordshire to an appointment in Bristol.

“I drive the M4 quite often,” he says. “There are too many roadworks and too many speed cameras – although I’ve been lucky lately.” Is his view of the M4 more in tune with Farage or Tatler? “I probably wouldn’t agree with either of them,” he laughs. “They’re just two different ends of the spectrum.” Sportingly, he thinks about this faintly ridiculous question for a moment. “Perhaps because it goes through Wiltshire I’d be more likely to agree with Tatler. But I drive all over the country, every motorway, I drive on all of them, and it’s just the same everywhere. The M4 is no posher than the rest of them, no posher than the M3 or the M6. None of them are as good as Scandinavian roads,” he adds. “Scandinavian roads are much better.”

“I don’t use the M4 very often,” says Elliot, 26, a project manager, on his way to Bath to visit his sister. “Farage’s comments are ludicrous, though. Maybe once upon a time he could’ve been a serious guy, but it’s over now, isn’t it?”

Jo Hopgood, 71, a retired boutique owner, is stopping for pasties at Greggs with her friend. “We’ve stopped because we’re hungry and we want a pee!” she says brightly. “And we like Greggs!” Today they are travelling from Surrey to Swindon for a painting workshop. “I like the M4,” she says. “And generally it’s pretty good. But at the moment it’s got the roadworks and speed restrictions, and getting out of London can be busy. But I do this route all the time, we have a house in Swindon, and it has lovely countryside and a couple of nice service stations. This is my favourite, because of the Greggs and the Costa, and my husband likes Cornwall pasties. You go through Windsor, which is lovely, and Bristol is fantastic, and there’s the outlet mall near Swindon. So I think, as motorways go, it’s a good one. It’s not such a car park as the M25, which I seem to spend half my life on.”

‘I think, as motorways go, it’s a good one. It’s not such a car park as the M25’ … Jo Hopgood on the M4. Photograph: Sarah Lee/Guardian

We set off again, discussing as we go the wider implications of traffic jams. It was Margaret Thatcher who claimed that the interminable jams of the M25 were a sign of success. Much of the congestion on the M4 lately has been due to construction work, which one might hope will herald an economic upturn and even improve travel to the west for Mr Farage’s future visits to Wales. Then again, one might also bear in mind the long-held theory that “roads cause traffic” – an argument backed up by a study in the American Economic Review, which found that road construction tends to lead to a proportionate increase in use.

We duck down through the charming streets of Marlborough and into Swindon, where we meet Kevin, a taxi driver accustomed to driving the M4, ferrying locals and businesspeople, making airport pick-ups. “You have Germans, Poles, Brits, everyone,” he says. “I wouldn’t say it was posh, and I wouldn’t say it was full of immigrants. I’d say it was just middle of the road. But I’d rather drive it than the M5. That’s so snarly.”

“It’s quite nice scenery driving along it,” concedes Dom Shand, 33, a First Great Western employee. “It’s very country life, lots of hills, lots of green.” And what does he think of Nigel Farage? “He has some decent points,” he says slowly, “but he’s a bit of a plonker, isn’t he? And those comments were shocking, weren’t they?”

They were indeed shocking. Also, statistically, Farage’s argument was flawed – his journey took a third longer than predicted, but immigrants do not make up a third of the population; rather, it’s around 13%, which means that they could account for around 36 minutes of the delay. Unless, of course, the M4 holds a particular appeal for immigrants.

I leave the photographer at Swindon station and continue west, wheeling over the Second Severn Crossing and into Wales. The motorway is black, near-deserted and whipped with heavy rain, and I finally reach Port Talbot seven-and-a-half hours after I set off. At least insurgent populist demagogues don’t have to conduct interviews. Admittedly, it does take me five hours to get back to London, but mostly the lanes are quite clear, the delays due to the nighttime roadworks underway and the 40mph speed restrictions as workers set out traffic cones. I peer at them as I pass: burly men in neon tabards, the air lit up by their breath. They’re holding up the traffic, it’s true, and they may very well be immigrants. On the other hand, Nigel, who else is going to mend the potholes?